A memorial for the (many) ages

Occasionally folks will ask how long the four faces of Mount Rushmore will still be visible. After all, even the sturdiest materials eventually succumb to wind and water.

Geologists estimate that the Mount Rushmore loses about an inch of rock every 10,000 years, so Mount Rushmore will likely be around for some time.

However, that’s not to say that the National Park Service is taking the threat of erosion lightly. Cracks in the stone can fill with water, which expands when it freezes. That could pry loose some stone, widen the cracks and open the way for more freezing/expansion later on. Also, seismic activity deep in the earth could cause movement of the massive granite slabs that make up Mount Rushmore.

Each fall, Park Service staffers don harnesses and examine the faces, filling the cracks as they go. For years, crews used Borglum’s recipe for sealing cracks -- linseed oil mixed with white lead and granite dust. Today, however, they use high-tech silicone caulk.

And hidden on the surface of the faces are a series sensors that measure minute rock movements over time. To protect the carving from lichens, smoke and other pollutants, a German firm, Alfred Kärcher GmbH & Co., voluntarily used its specialized spray equipment to pressure wash the entire memorial in 2005.